Macroscopic, continuous layers of strained crystalline materials tend to relax plastically upon heating to a high enough temperature provided the total strain energy exceeds a critical value. Plastic relaxation refers to a mechanism whereby crystalline defects are introduced into the strained layer, which serve to reduce the total strain within the film. These crystalline defects themselves require energy to form and thus only do so when the total strain energy within the layer is greater than the formation energy of the strain-relieving defect. This criterion is what defines the critical strain value of a system. The critical strain value is dependent on the type of defects formed and the mechanical properties of the crystal.
In the case of compressively strained SiGe layers grown on a Si template the predominate type of strain-relieving defect formed is a misfit dislocation. Misfit dislocations move through the crystal after forming in a manner that is dictated by the nature of the stress tensor at the instantaneous position of the dislocation. For strained SiGe layers grown on a Si substrate, where the Si thickness is many times that of the SiGe layer, the dislocations glide upwards through the SiGe layer. If the Si substrate is thinner than the SiGe layer, the nature of the forces acting on the dislocations is such that the dislocations will glide downward onto the Si layer. The latter phenomenon is described in an article by Y. H. Lo entitled “New approach: to grow pseudomorphic structures over the critical thickness”, Appl. Phys. Lett., Vol. 59, No. 18, October 1991, pp. 2311-2313.
Use of silicon-on-insulator (SOI) substrates with a thin Si layer above a buried oxide layer were considered for making relaxed SiGe layers with low-defect density by forcing the defects to be driven into the underlying thin Si layer. For this phenomenon to occur the buried oxide layer had to behave viscously so that the Si layer would act like a free layer. The problem with this prior art approach is that a highly defective top SiGe layer is exchanged for a highly defective underlying Si layer, and neither scenario is acceptable for use with modern complementary metal oxide semiconductor (CMOS) applications.
In view of the prior art mentioned above, there is a need for providing a new and improved method in which a high-quality, substantially relaxed SiGe layer can be formed atop an SOI substrate whereby the highly defective underlying Si layer is eradicated after it serves as a defect sink for producing the substantially relaxed SiGe layer.